Method of Fabricating Layered Nano-Carbonate Used for Medium-High Temperature CO2 Sorbent

ABSTRACT

A technique of fabricating a medium-high temperature CO 2  sorbent of layered nano-carbonate is provided. A CO 2  sorbent is fabricated. The sorbent captures CO 2  at a medium-high temperature above 600° C. Calcium acetate is introduced for making a nano-scale layered double hydroxide (LDH). The layered structure is used for templated synthesis. The sorbent has an initial conversion rate above 90%; and the conversion rate remains the same even after 100 times of carbonation/de-carbonation cycles.

TECHNICAL FIELD OF THE DISCLOSURE

The present disclosure relates to a CO₂ sorbent; more particularly, relates to introducing calcium acetate (Ca(Ac)₂) for fabricating a nano-scale layered double hydroxide (LDH) of Ca(Ac)₂—Al—CO₃, where layered structure of the LDH is used in templated synthesis to obtain a medium-high temperature CO₂ sorbent for capturing CO₂ at a temperature above 600° C.

DESCRIPTION OF THE RELATED ARTS

Calcium oxide (CaO) based material can be used for capturing CO₂ at a medium-high temperature above 600° C. Because the long-term durability through regeneration cycles is a crucible feature of solid sorbent for capturing CO₂, reaction temperature of CO₂ and metal oxide as well as regeneration temperature of the metal carbonate become essential. Among a few metal oxides, CaO has the feature of both the CO₂ reaction temperature and the decomposition temperature of metal carbonate located between 600° C. and 800° C., which is a good sorbent applied for high-temperature conditions. Calcium-based oxides can be divided into two categories: raw material and immobilized material. The former means those containing alkaline metals like limestone, calcite and artificial pretreated CaO. The later means those synthesized with powders or structural materials of alkaline metal oxide like zeolite with high surface area modified with calcium (Ca)- or magnesium (Mg)-oxide. CaO has a high CO₂ conversion rate because the carbonation product of calcium carbonate (CaCO₃) is a thermodynamically stable material. However, some reactions between CaO and CO₂ are irreversible. Besides, CaCO₃ may easily be sintered and porous structure may thus be destroyed. Usually, surface area and pore volume of CaO are increased to reduce in regeneration cycles. Hence, for increasing surface area, radiuses of the raw materials are minimized or materials are immobilized on a high surface support.

Some methods for size control are revealed, like precipitating CaCO₃, wet-impregnating calcium ions (Ca⁺²) on silicon dioxide (SiO₂) or synthesizing a composite material of CaO—Al₂O₃. Or, Organic additive can also be used to enhance synthesizing colloid solution containing Ca⁺². Another solution is to apply nano Ca⁺² gel on porous material of polymethylmethacrylate (PMMA). In recent researches, co-precipitation is used. Magnesium ions (Mg⁺²) in a structural material of layered double hydroxides (LDH) are replaced with calcium ions (Ca⁺²) to obtain a template for making powders or films with nano CaO structures. This kind of materials can be used to capture CO₂ or other selected gas above 400° C. Yet, the above prior arts do not introduce Ca(Ac)₂ for templated synthesis with layered structure in obtaining Ca/Al(aluminum) carbonate for capturing CO₂ at a temperature above 600° C. Hence, the prior arts do not fulfill all users' requests on actual use.

SUMMARY OF THE DISCLOSURE

The main purpose of the present disclosure is to fabricate a synthesized nano-scale layered composite material of Ca(Ac)₂—Al—CO₃ having dense pores.

The second purpose of the present disclosure is to provide a nano-scale LDH of Ca(Ac)₂—Al—CO₃ with Ca(Ac)₂ introduced, where the layered structure is used for templated synthesis to obtain a sorbent for capturing CO₂ above 600° C.

To achieve the above purposes, the present disclosure is a method of fabricating a medium-high temperature CO₂ sorbent of layered nano-carbonate, comprising steps of: (a) obtaining reactants of Ca(Ac)₂, aluminum nitrate (Al(NO₃)₃) and sodium carbonate (Na₂CO₃) to process a reaction with sodium hydroxide (NaOH) added to control alkalinity; (b) obtaining a layered material of calcium/aluminum (Ca/Al) carbonate after the reaction; (c) calcining to remove anions and acetate in the layered material; and (d) synthesizing a nano-scale layered composite material of the Ca/Al carbonate (Ca(Ac)₂—Al—CO₃). Accordingly, a novel method of fabricating a medium-high-temperature CO₂ sorbent of layered nano-carbonate is obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure will be better understood from the following detailed description of the preferred embodiment according to the present disclosure, taken in conjunction with the accompanying drawings, in which

FIG. 1 is the flow chart showing the preferred embodiment according to the present disclosure;

FIG. 2 is the XRD pattern showing the Ca(Ac)₂—Al—CO₃ before being calcined;

FIG. 3 is the table showing the ratios of Ca and CaO in the Ca(Ac)₂—Al—CO₃;

FIG. 4 is the table showing the CO₂ absorption of the Ca(Ac)₂—Al—CO₃;

FIG. 5 is the figure showing the CO₂ conversion rates for 40 times cycles; and

FIG. 6 is the table showing the CO₂ conversion rates for 100 times cycles.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The following description of the preferred embodiment is provided to understand the features and the structures of the present disclosure.

Please refer to FIG. 1 to FIG. 3, which are a flow chart showing the preferred embodiment according to the present disclosure; an XRD pattern showing Ca(Ac)₂—Al—CO₃ before and after being calcined; and a table showing ratios of Ca and CaO in the Ca(Ac)₂—Al—CO₃. As shown in the figures, the present disclosure is a method of fabricating a medium-high-temperature CO₂ sorbent of layered nano-carbonate, comprising the following steps:

(a) Obtaining reactants and adding NaOH 11: Calcium acetate (Ca(Ac)₂), aluminum nitrate (Al(NO₃)₃) and sodium carbonate (Na₂CO₃) are obtained as reactants to process a reaction and sodium hydroxide (NaOH) is added to control alkalinity.

(b) Forming layered material 12: A layered material of calcium/aluminum (Ca/Al) carbonate is formed after the reaction, where the layered material of Ca/Al carbonate is a layered double hydroxide (LDH) comprising two cations and anions. Thus, a template is obtained with a layer of oxide octahedron formed by cations of Ca and Al and with layers containing carbonate (CO₃ ²⁻) and hydroxide (OH⁻).

(c) Calcining 13: Anions and acetates in layers are removed by calcining while the ratio of Ca to Al is changeable.

(d) Synthesizing composite material 14: Thus, a nano-scale layered composite material having Ca/Al carbonate is synthesized.

Therein, the LDH is synthesized through calcining to form a Ca—Al oxide solid solution containing micro-crystals of Ca(OH)₂ and CaCO₃. When the ratio of Ca:Al is 7:1, the Ca(Ac)₂—Al—CO₃ is synthesized with a pore volume of 0.019 cc/g. The Ca(Ac)₂—Al—CO₃ has micro-/mesopores with an average radius between 2 and 6 nanometers (nm). The Ca(Ac)₂—Al—CO₃ has an initial conversion above 90% and remains the 90% conversion rate even after many times of carbonation/de-carbonation cycles.

Amount of calcium oxide (CaO) in the Ca(Ac)₂—Al—CO₃ is important to carbon dioxide (CO₂) absorption. In FIG. 3, when the ratio of Ca:Al is a value of 1˜5 or 7˜20, CaO contained in the Ca(Ac)₂—Al—CO₃ is 48˜64% or 66˜77%, which is calculated by Ca % determined by inductively coupled plasma (ICP) spectroscopy.

Please refer to FIG. 4 to FIG. 6, which include a table showing a CO₂ absorption of Ca(Ac)₂—Al—CO₃; a figure showing CO₂ conversion for 40 cycles; and a table showing CO₂ conversion for 100 cycles. As shown in the figures, on using the present disclosure, a synthesized Ca(Ac)₂—Al—CO₃ captures CO₂ at a high temperature between 400° C. and 800° C. with an initial conversion rate higher than 96% while the conversion still remains high even after many times of carbonation/de-carbonation cycles. In FIG. 4, the Ca(Ac)₂—Al—CO₃ captures greater amounts of CO₂ with a higher Ca:Al ratio at 750° C. When the ratio is 1-20, the amount of captured CO₂ is 13.4˜66.9%. But, when the ratio of Ca:Al is higher than 7, crystalline phase of the synthesized material may become complex and cost may become high. In FIG. 5, a CO₂ conversion after 40 times of cycles at 750° C. is 95%. In FIG. 6, a CO₂ conversion rate after 100 times of cycles at 750° C. still remains 90% (remains 65% even after 200 times of conversion, not shown in the figure). After 100 times of CO₂ capturing cycles at 750° C. through Ca(Ac)₂—Al—CO₃, 47 times amount of CO₂ based on using sorbent weight is captured.

Thus, the present disclosure introduces Ca(Ac)₂ for fabricating a nano-scale LDH of Ca(Ac)₂—Al—CO₃, where the layered structure is used for templated synthesis to obtain a medium-high temperature CO₂ sorbent for capturing CO₂ above 600° C. with an initial conversion above 90%, which conversion remains the same even after 100 times of conversion.

To sum up, the present disclosure is a method of fabricating a medium-high-temperature CO₂ sorbent of layered nano-carbonate, where Ca(Ac)₂ is introduced for fabricating a nano-scale LDH of Ca(Ac)₂—Al—CO₃; and the layered structure is used for templated synthesis to obtain a medium-high temperature CO₂ sorbent for capturing CO₂ above 600° C. with an initial conversion rate above 90%, which conversion rate remains the same even after 100 times of conversion.

The preferred embodiment herein disclosed is not intended to unnecessarily limit the scope of the disclosure. Therefore, simple modifications or variations belonging to the equivalent of the scope of the claims and the instructions disclosed herein for a patent are all within the scope of the present disclosure. 

1. A method of fabricating a medium-high temperature CO₂ sorbent of layered nano-carbonate, the method comprising: obtaining reactants of calcium acetate (Ca(Ac)₂), aluminum nitrate (Al(NO₃)₃), and sodium carbonate (Na₂CO₃) to process a reaction with sodium hydroxide (NaOH) which is added to control alkalinity; obtaining a layered material of calcium/aluminum (Ca/Al) carbonate after said reaction; calcining to remove anions and acetate in said layered material; and synthesizing a nano-scale layered composite material of said Ca/Al carbonate (Ca(Ac)₂—Al—CO₃), wherein said layered material is a layered double hydroxides (LDH) comprising two cations and anions; wherein a layer of oxide octahedron is obtained with said cations of Ca and Al; wherein said layered material has layers containing carbonate (CO₃ ²⁻) and hydroxide (OH⁻); wherein said octahedron is used as a template to remove said anions and said acetate; wherein said Ca(Ac)₂—Al—CO₃ has micro-/mesopores; and wherein each of said pores has a radius between 2 and 6 nanometers (nm).
 2. The method according to claim 1, wherein said Ca(Ac)₂—Al—CO₃ has a ratio of Ca to Al (Ca:Al) between 1 and
 5. 3. The method according to claim 2, wherein said Ca(Ac)₂—Al—CO₃ has a ratio of calcium oxide (CaO) between 48% and 64%.
 4. The method according to claim 1, wherein said Ca(Ac)₂—Al—CO₃ has a ratio of Ca:Al between 7 and
 20. 5. The method according to claim 4, wherein said Ca(Ac)₂—Al—CO₃ has a ratio of CaO between 66% and 77%.
 6. The method according to claim 1 wherein said Ca(Ac)₂—Al—CO₃ has a pore volume of 0.019 cc/g when being synthesized with a 7:1 ratio of Ca:Al.
 7. The method according to claim 1, wherein said Ca(Ac)₂—Al—CO₃ captures carbon dioxide (CO₂) at a temperature between 400° C. and 800° C. with a 96% initial conversion rate.
 8. The method according to claim 1, wherein said Ca(Ac)₂—Al—CO₃ captures 47 times of used sorbent weights of CO₂ after 100 times of carbonation/de-carbonation cycles.
 9. The method according to claim 1, wherein said Ca(Ac)₂—Al—CO₃ has an initial conversion rate of CO₂ above 90%.
 10. The method according to claim 1, wherein said Ca(Ac)₂—Al—CO₃ has a conversion rate of CO₂ above 90% after 100 times of carbonation/de-carbonation cycles. 